The present invention relates to interferometry methods, and more particularly to interferometry methods using illumination sources with varying wavelengths, such as tunable lasers.
Phase-shifting interferometers are known to those skilled in the interferometry art. Exemplary systems are available from both Zygo Corporation of Middlefield, Conn. (www.zygo.com) and Veeco Instruments Inc. of Woodbury, N.Y. (www.veeco.com). Typically, these systems use either a single-wavelength laser or “white” (i.e. polychromatic) light.
Single-wavelength interferometers have traditionally used gas lasers such as helium neon (HeNe). More recently, the single-wavelength systems have begun to use diode lasers in which the output wavelength can be controlled by controlling the temperature and forward current of the laser. Accordingly, single-wavelength systems typically do not suffer from wavelength drift and/or jitter.
White light systems utilize a broadband light source, either alone or in conjunction with a narrow-band interference filter. These systems also typically do not suffer from such degradation issues as wavelength drift.
More recently, multiple-wavelength interferometer systems have begun to enter the marketplace. These systems typically include a tunable laser to achieve their multi-wavelength performance. Many types of suitable tunable lasers exist, and examples include dye, titanium sapphire, Alexandrite, distributed feedback diode, distributed Bragg reflection diode, and external cavity diode. At present, the tunable diode lasers are the most cost effective in commercial applications. Unfortunately, the wavelength output of diode lasers varies as a function of temperature and forward current (and other parameters). Modern electronic control circuitry, while advanced, is unable to regulate the temperature and forward current (and other parameters) adequately to stabilize the laser so that the wavelength does not drift or jitter. For example, external cavity tunable lasers depend on the electromechanical movement of the external cavity, which sometimes contains a grating and/or a mirror. There is a tendency for these systems to drift and jitter in wavelength as the electronic control attempts to stabilize the laser by modifying the temperature, forward current, and position of the grating.
Interferometers are sufficiently sensitive so that wavelength variation on the order of one hundred (100) picometers or less can translate into potential phase error in an unequal path length interferometer. Accordingly, multiple-wavelength interferometers and laser radar systems utilizing such lasers have resulted in less than desirable performance due to laser drift and jitter.